Electronic amplification



y 1945- c. B. FISHER 2,379,513

ELECTRONIC AMPLIFICATIQN Filed June 10, 1942 4 Sheets-Sheet 1 SIGNALSHAPERS AMPLIFIERS s I EA A .5, l o o 5 9/,

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C. B. FISHER ELECTRONIC AMPLIFICATION Filed June 10. 1942 4 Sheets-Sheet5 57gfff- 1.2

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CJar/e: 5. FI r/I ATTORNEY ELECTRONIC AMPLIFICATION Filed June 10, 19424 Sheets-Sheet 4 74 7/ A MP.

CARRIER IMPEDANCE SOURCE 72 UNIT LOAD 75 AME ISOLATION CIRCUIT /J7SIGNAL 4 SHAPERS AMP. 3 AMP. AMP.

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ATTORNEY Patented July 3, 1945 UNITED STATES PPATENT OFFICE ELECTRONICAMPLIFICATION I Charles B. fisher, Washington, D. C.

Application June .10, 1942, Serial No. 446,436

Claims.

This invention relates to electronic amplifiers and more particularly toa novel method of and means for substantially increasing the emciency ofoperation of vacuum tube power amplifiers.

Th present invention is generally applicable to audio frequency andradio frequency circuits incorporating linear power amplification. Theincreased efficiency of power amplification by the invention may be usedfor amplifying carrier waves, modulating signals, or signal-modulatedcarrier waves, for the generation of radio waves by the oscillationprinciple, and so forth. The well known class B type amplifier is usedwhere high efficiency linear power' amplification is desired. However,the plate-circuit efiiciency of such amplifiers depends upon themagnitude of the signal voltage, since the efilciency is substantiallyproportional to the ratio of the signal voltage output to the applieddirect-current voltage. Where the signal is of variable amplitude, theaverage efiiciency of the power amplification by such prior-art systemsis relatively low. Where, however, the signal is at a constant maximum,higher efiiciency values 'for the class B and C amplification result,ranging in practice from to In accordance with the present invention,

a substantially higher power-conversion efficiency is aiforded than byclass B linear amplification. The invention is particularly useful forsignalmodulatedcarrier-wave amplification, where the modulating signalis of a varying amplitude, as occurs for example in-speech. A pluralityof parallel amplifying paths or branches are provided for individuallyamplifying separate portions of the impressed signal. Each amplifier isresponsive only to a predetermined magnitude level or portion of theimpressed signal, thereby selectively amplifying sections of the signal.The outputs of the amplifiers are combined to reconstruct the originalimpressed signal wave form. Each of the amplifier branches is preferablybiased for class B amplification, as will be hereinafter set forth indetail. Thus, each of I the parallel amplifier branches is operated at asignal value corresponding to its maximum capacity, and correspondinglyamplifies its signal portion at the maximum operating efficiency theamplifier branch is capable of. When no signal portion is impressed onan amplifier branch no direct current energy isdissipated therein. The

' overall result for the system is accordingly an amplificationefflciency substantially higher than heretofore possible in practice.Practical ex- 5 amples and applications thereof are described in detailhereinafter.

It is well known that the maximum power-handling capacity of any vacuumtube is ordinarily limited by the maximum permissible values of itsaverage and peak plate currents, its peak plate voltage, and its averageplate power dissipation capacity. If the efficiency of its average platepower dissipation is for example increased from 50% to as is feasible bythe invention, the power-handling capacity of the vacuum tube isdoubled. Since, in accordance with the present invention, the averageplate current is much closer to the peak plate current than inconventional amplifiers, it has been found that in practical embodimentsof the invention such double power output from a given tube complementis readily obtained as compared to conventional circuits. This is ofsubstantial advantage in. high power installations from the standpointof cost.

25 In lower power mobile radio transmitting equipment a substantialreduction in space and weight requirements is aiforded. The inventionsystem results, for amplifiers of about one-third, and for transmittersof about one-half the conven- 30 tional space and weight requirements.This is due mainly to a substantial reduction in the heat dissipation ofthe tubes and for other reasons to be later set forth. The presentinvention has been. found more economical than conventional arrangementsfor amplifiers or transmitters having power outputs greater than aboutwatts. The principles of the invention are useful for all commercial andmilitary communication services, including point-to-point andtrans-oceanic 4o telephony, civilian and military aviation, and

other services in general,

These and further advantages, capabilities and objects of the presentinvention will become more apparent from the following detaileddescription 45 of preferred embodiments thereof, illustrated in theaccompanying drawings, in which:

,Fig. 1 is a schematic diagram illustrating the principles of theinvention; Fig. 2 is a circuit diagram of one embodiment which theinvention 5 may assume in practice; Figs. 3 to 9 are curves used indescribing the principles and operation of the invention; Figs. to 12are curves representing efilciencies of amplification for varyingnumbers of amplifier branches; Fig. 13 is a schematic outputimpedance-matching) diagram for the form of the invention shown in Fig.2; Fig. 14 is a circuit diagram of a modified output coupling unit, forthe circuit oi Fig. 2; Figs. 15 to 17 are .curvesused illustrating theoperation of the modified form of the invention corresponding to Fig.14; Fig. 18 is a schematic diagram of a ifurther application of theinvention; Figs. 19 to 22 are curves illustrating one mode of operationof the system of Fig. 18; Fig. 23 is a block diagram of an oscillatoremploying the circuit of Figure 2.

Fig. 1 schematically indicates the principle of the present invention.Signal source of a wave form S is shown applied to the input of aplurality of signal shapers" 2|, 22, 23. A sinusoidal wave shape is usedfor S for simplicity of presentation. The invention is of courseapplicable to any wave form. The signal shapers are units arranged todivide the input signal S into separate components as A, B, Crespectively. Such division is efiected by suitable circuit arrangementsto be described, whereby predetermined sections of the wave S are passedthrough the respective branches I, II, III of the system. Theg'is'anches have individual amplifier units 24, 25,

The wave forms A, B, C passed through amplifiers 24, 25, 26, result fromthe corresponding introduced sinusoid sections A, B, C for one form ofcircuit coupling. The outputs of the amplifiers 24, 25, 28 are impressedupon an impedance unit 21, across which the signal components A, B, Ccombine substantially to reconstruct the original wave shape. There-formed waveshape S is shown as containing harmonic components. Formany applications a minor harmonic content is not objectionable.However, by use of suitable filters or degenerative feed-back, suchharmonics may be reduced to any extent desired. The load circuit isconnected to impedance unit 21 and receives the signal indicated at S",corresponding to the input wave form S, and without the harmonics ifdesired.

Fig. 2 is a circuit diagram of one form which the invention may assumein practice. This form comprises the three branches I, II, III. The

input circuit of each of these branches is connected to the signalsource 20. The outputs of the branches are coupled to a transformer unit30. The output of transformer 30 is connected to load 3|. Load 3| maybea suitable translating device, such as a radiation circuit, as will beunderstood by those skilled in the art. The branch circuits arepreferably designed in push-pull arrangement, namely with opposed pairsof tubes for each portion. The signal shapers com rise triodes 32, 33for branch I; 34, 3 5 for branch II; and 36, 31 for branch III. Theamplifiers similarly comprise triode paths 38, 39 for branch I; 40, 4|for branch II; and 42, 43 for branch III. Triodes have been shown forsimplicity. However, pentodes or other suitable vacuum tubes may insteadbe used.

The amplifier units are connected in a conventional manner, and arecoupled to the outputs of the signal-shaper tubes by the indicatedresistance-capacity coupling. Input resistance un ts 44 to 49 areprovided in pairs for the respective branches. The resistances are ofsuch values as to give the required voltages in each of the threebranches into which the circuit is divided. The amplitude of the voltageapplied to branch I is controlled by resistances ll, to branch II, byresistances 43, 41; and to branch III, by resistances 43, I9.Direct-current grid-bias potentials 30, BI, 52 are provided for the.gridcircuits of the respective signal-shaper portions. The division of theapplied signal from source 20 to the grid electrodes 0! the signalshapers of the respective branches is accordingly predetermined by thevalues 01' the respective resistances 44, 45 and bias 50 for branch I;resistances 46, 41 and bias SI for branch II: and resistances, 43 andbias 52 for branch III.

It is thus feasible to divide the applied signal into threepredetermined sections which, when amplified and recombined, reform theoriginal wave shape. In accordance with the invention such division ofthe input signal is made so that each of the resultant waves is of suchform and amplitude that it is transmitted through the amplifier units ofthe respective branches with substantially higher efliciency than in theoriginal wave shape. The overall efilciency obtained is accordinglysubstantially higher than heretofore possible with prior circuits. Thiscondition can be derived in a number of ways to attain the objects ofthe present invention. One of such methods which has been foundsatisfactory will now be set forth in detail.

For the three-branch embodiment illustrated in Fig. 2, the ratio of themaximum power outputs of the respective branches is designed to be 1:224

for the branches I, II, III. That is to say,'the load currents from thethree branches are in the ratios l:1.4:2. These outputs are applied tothe primary winding 53 of transformer 30. The output of branch I isshown connected across the outer terminals of primary winding 53; theoutput of branch II, across an intermediate section of winding 53; andthe output of branch III, to the central section of winding 53. The tapson the output-transformer winding 53 are selected to suit the respectivebranches so that each pair of output tubes connects into an appropriateload impedance. Also, each of the output tube pairs 38, 33, 40, 4| and42, 43 is biased to the point where the anode current thereof isnormally reduced to zero. The amplifier units are accordingly operatedas class B or class C amplifiers. The tube pairs comprising the signalshapers are essentially driver stages for the respective amplifiers, andare used to shape the signals impressed thereon from source 20 for thegrid electrodes of the power-amplifier tube units.

Figs. 3, 4 and 5 illustrate preferred wave forms of the voltages passedby the signal-shaped portions to the inputs of their corresponding amplifiers. The principles of the invention are for the sake of simplicityillustrated in connection with an original signal having a sinusoidalwave shape. Figs. 3, 4 and 5 are curves representing a manner in whichthe original sinusoidal signal S is divided by the signal shaperportions of the three branches I, II, III. The forms of the waves inthese figures are drawn to have the same amplitude, which condition is,however, not essential. In a circuit in which it is desired that no gridcurrent shall flow in the power amplifier tubes, the maximum value ofthe waveimpressed upon the grid electrodes of the respective powersamplifier units is the value of their respective grid-bias voltage, orthe value which will result in maximum permissible plate current for thetubes, whichever is less. However, in a circuit where grid current ispermissible. the peak grid drive impressed upon the amplifier units isadjusted so that the maximum plate current is obtained.

The invention takes advantage or the well known fact that avacuum tubeamplifier unit will operate at its maximum effective emciency, when theamplitude of its input or driver signal is at the peak value and theplate current of each or the tubes is transmitting its Peak currentvalue. In a conventional class B or class C amplifler an efllciency ofthe order of 90% is realized at the moment when it is delivering. itsinstantaneous peak output, this efllciency being the ratio of the peakoutput voltageto the D. C. input voltage for the tubes. Thus, if aconsiderable portion of the output energy is transmitted while the tubesare operating at such peak current points, then the whole output powerof the tubes is obtained at a relatively high efliciency. In accordancewith the present invention, the respec-' tive branches are designed withsignals impressed upon their amplifier units to operate close to theirpeak current ratings, resulting in a higher operating efliciency for thesystem than hereto- Thus, the load-current ratio for the respectivepower amplifier units for the three branches would be in the ratio ofl:0.7:0.5 for branches I, II, III. The load currents for the system aredivided among the three branches in this manner. For the sine-wavesignal S, branch I will pass signal section A as indicated in Figs. 1and 3. Resistances 44, 45 and bias 50 of branch I are designed so thatit will pass all portions of the input signal S below 0.5 in relativeamplitude; section A thus corresponding to one-half of the appliedsignal amplitude. The next 3 db. amplitude range, namely from 0.5 to 0.7is transmitted by branch II, corresponding to signal section B shown inFigs. 1 and 4. Resistances 46, 41 and bias 5| of branch II are designedso that driver tubes 34, 35 pass only a signal corresponding to sectionB from the impressed signal, and

'no other portion. Finally, the third 3 db. signal section,corresponding to C and lying between 0.7 and 1.0 in amplitude, istransmitted only by branch III. Resistances 48, 49 and bias 52 forsignal-shaper tubes 36, 31 of branch III are designed to pass only thecorresponding 3 db. current section C.

It is to be understood that the invention is not limited as to thebranches or to the signal intervals. Thus, if 3 db. intervals are usedbetween adjacent branches, and there are more than three branches, therelative current values below 0.5 would be handled by more than onebranch. Also, the interval ratio between the signal sections for thebranches may be as much less than 3 db. as desired, with the use of acorrespondingly larger number of branches. Furthermore, the intervalratio of the signal sections between adjacent branches need not be thesame as that between any other adjacent branches. A high efliciencysystem still results with a ratio interval of more than 3 db.

The particular ratios for the signal sections are determined inpractical design by the ratings of available tubes used, and also by theaverage configurations of the wave shapes to be amplifled' It the volumerange is large, and during the major portion or the time the wave has .arelatively small amplitude, then there preferably should be a relativelylarge interval between the maximum current-capacity oi the highestbranch and the adjacent branch. Also, there preferably should be arelatively small interval between the lowest- Dower branch and thebranch adjacent to it, with the intervening intervals havingintermediate values. Such a general scheme is preferred for a signalamplitude-modulated carrier wave; On the other hand, where the range-oiamplitude variation is small, and the wave is at or near the peak valuefor the greater part of a cycle, then the interval between thehighest-power branch and the adjacent branch is preferably relativelysmall, and the lowest-power branch should preterably handle a.vrelatively large current. Other factors remaining equal, there isgenerally more justiilcation for a relatively large number of branchesin the first category referred to herein,

and for a relatively small number, possibly only two, in the lattercase.

The grid biases 50, II, signal-shaper circuits for push-pull drivertubes, are adjusted to provide conventional class B operation. Also, therespective gains of the signalshaper drivers'are adjusted in conjunctionwith the plate coupling resistances so that the peak power is used tocause plate-current saturation in their respective coupledpower-amplifier units. As the input signal from source 20 progressesfrom 0 phase angle to 30, the circuit'is such that only branch Iconducts, the other driver tubes being biased too far down to permit anyplate current to then flow in the other branches. Fig. 3 shows therelative signal thus passed by the signal shaper section of branch I.

Fig. 6 shows the corresponding current passing 1 40, 4| at the same timethen rapidly rise to their peak current, reaching the 0.707 relativevalue at a phase angle of 45 as seen in curve B in Fig. 7. The signalsection B of Fig. 4 is seen passing into driver tubes 34, 35'.betweenthe 30 and 45 phase angular relations, and levelling ofi at 45 with therelative signal grid voltage at one-half the peak value for signal S. Atthis point also, the branch III tubes start conducting, and the outputcurrent of branch II rapidly drops off as a result of the much higherload impedance thereupon presented to the branch II output.

Curve C' of Fig. 8 illustrates the relative load current passing throughoutput tubes 42, 43 of branch III, reaching its maximum current ratingat the phase angle position, and decreasing to zero at about When signal0' reduces to zero, there results a reduction in the load impedancereflected into power-amplifier tubes 40, 4| of branch II, which almostimmediately start to conduct at their maximum cur rent value, asindicated in Fig. 7. As it reduces in valuewith the advance of the phaseangle, the plate current B of the tubes of branch II ap- 52 for therespective proaches zero at about the 150 position 'Iher upon, the loadimpedance reflected into the output of branch I is rapidly reducedpermitting amplifier tubes 3|, 3| to conduct and complete the wave formA of Fig. 8 at 180. It is to be understood that the relativesignal-shaping actions resulting in the signal sections for the branchesare correlated with the output currents of the branches, and therespective reflected impedance emciently to produce the correspondingwave form therefor.

Th functioning of the output transformer in reflecting theresistancesbetween the outputs of the respective branches may be better understood-by reference to the schematic diagram of the system represented by Fig.13. This schematic circuit is an idealized representation of the threebranches of Fig. 2. The impedances of the respective branches areindicated at 21, 2 2;. The load resistance is Zn. The signal voltage of,the respective branches are indicated as or, ea, as. The impedances ofthe respective sections of the transformer coupled to the branches areindicated as Zr, Z2, Z3. The switches indicated at S1, S2, represent theoperation of the signal shaping sections of the respective branches.Assume switch Si closed and switches S2 and S3 open. The voltage at tapZ: is equal to l Z1 1+ i Now, if switch S2 is closed, and the voltage ofZ2 delivered by branch II is slightly greater than this, then thevoltage appearing at Z1 will be raised slightly and the current flowinginto 21 is no longer the former value of 31 +Z1 but is equal to i+ 2where ea is the back voltage at the tap Zl. The corresponding current iiin branch I is reduced as voltage ed increases. When voltage es becomesequal to e1, current ii in branch I becomes zero. Thus, the impedance ofthe transformer winding facing branch I approaches infinity. This sameprocess is repeated for the transition between branch II and branch III.

The output signals from branches I, II, III impressed upon the primary53 of the output transformer 30 combine to induce the resultant signal Sin the secondary winding 54, and at load 3i. Fig. 9 shows the resultantsignal S, substantially corresponding to the sinusoidal input signal Sin wave shape. The signal components A; B, 0., corresponding to thecurves of Figs. 6, '7 and 8 are integrated to form the signal S. Thewave form of S has a high frequency harmonic content. As the number ofbranches for the system is increased above three, the order of theharmonic content correspondingly increases. However, the amplitude ofthe corresponding harmonic decreases substantially inverselyproportionately therewith. It is to be noted that for most applicationsto which the present invention is indicated, such amount of distortion,in practical designs lying between 6% and 14%, is tolerable. In practiceit is known that a distortion content of 20% is tolerable for commercialand military communication services, such as pOint-to-point andtrans-oceanic radio telephony, civil and military aviation, tank radio,and other military and naval applications.

v is equal to the ratio of Where low-distortion systems are required,the well-known principle of degenerative or negative feed-back may beemployed to reduce the harmonic content of the output wave form to'anydesired value. The application oi.negative feedback is indicated in theform of the invention disclosed in connection with Fig. 18. It is to benoted that the frequency range required to be transmitted with littlediscrimination between the plate circuit of the signal shaperor drivertubes and the input circuit of the power amplifier tubes is more strictthan for other systems of amplification in view 'of'the non-sinusoidalwave form of the signal sections. In practice. the use of'a plurality ofbranches in the system design does not involve difficulties as tounequal amplification. In the first place, all branches are subject tothe same gain-determining factors. such as temperature, humidity. aging,atmospheric pressure, etc. In any event, the use of negative feed-backcounteracts any distortion which might tend to occur in this connection.

The signal-shaping portion of the respective branches may be provided indifferent form than that illustrated in Fig. 2. For example,.threeelectrode gaseous conducting trigger" tubes of the 'Ihyratron typecould readily be adopted for this purpose. The grid electrodes of theThyratrons are biased so that conduction therethrough would start at thedesired phase angle of the input wave. Also, in order that the Thyratronwould become extinguished at the required phase angle of each cycle, itis necessary, for example, to supply them with a relativelyhigh-frequency anode power supply, such as -kilocycle current. In such acase, the combined output current must be rectified so as to pass onlythe signal frequency, rejecting the 50-kilocycle signal and theassociated high frequency components. On the other hand, it is alsofeasible to set up the signal components of the input wave by means ofmechanical contacts.

With such alternative arrangements it is possible to obtain a resultantwave form somewhat more preferable than that illustrated at S in Fig. 9.A still further arrangement for the signal shaping action is to use theinput circuit of the power amplifier tubes of each branch to shape thesignal pulses. This may be accomplished by adjusting the amplitude ofthe input wave, and the grid bias of the power amplifiers, so that theplate current cut-ofi is obtained at the requisite signal phase angle,and the peak grid voltage is of the required value to produce maximumpermissible plate current, or by using the plate current of branch I toreduce the bias of branch 11 power amplifier tubes and thereby bringthem into operation at the required time. What is significant in thepresent invention is the resultant division of the original signal waveform on an amplitude basis into component sections by suitable means,namely by the signal shapers"; which component signal sections can beamplified and transmitted by separate amplifiers each operating atrelatively high efiiciency.

Fig. 10 illustrates the power efilciency of the invention system fordifferent numbers of branches. The curve of Fig. 10 is based upon 3 db.intervals between the branches. The efficiency is plotted on thevertical coordinate and the peak output signal voltage to the directcurrent plate voltage, expressed in percent. The horizontal coordinateis a, percentage scale. One hundred represents the maximum amount ofplate current of the 100% modulated wave.

where the current is 0.707 of the peak value, and r then would risealong the line HJ. In practice, however, the lower-power branchcontinues for a short time to deliver power at a relatively highefliciency, and the resulting efliclency follows that shown by thecurved line GJ. A similar process is involved as additional branches areadded to the system, resulting in the following efllciency char.acteristics:

Efliciency curve for three branches=OEGJ Efficiency curve for fourbranches=OCEGJ Efficiency curve for five branches=0ACEGJ Efliciencycurve for ten branches -OKACEGJ If an input wave having a sinusoidalform is impressed on systems with different numbers of branches,different average efilciencies of transmission will result. The mannerin which the efiiciency thus varies is shown by Fig. 11. This curve isreadily obtained by averaging over one cycle the efficiency oi a sinewave with a peak value coinciding with 100 on the horizontal scale ofFig. 10. Fig. 12 shows a similar efiiciency characteristic for a signalcorresponding to a sinewave signal-modulated carrier signal. It is to benoted that Figs. 11 and 12 apply only to systems wherein there is a 3db. interval between the respective branches. It is to be understoodthat the efficiency values indicated herein can be appreciably improvedby a suitable adjustment of the maximum current ratio between thebranches, as hereinabove set forth. The curves of Figs. 11 and, 12indicate the large improvement in efficiency which the present inventionmakes possible. The data represented by these curves has been verifiedexperimentally, and in all cases the efficiencies measured have beenslightly greater than these theoretical curves show. This may beexplained by the slight variations in the shapes of the curves joiningthe points of peak efiiciencies in Fig. 10. Y

The usual efficiency of a conventional class B amplifier is about 50%for a sine wave output. A class C amplifier has a higher efilciency thanthe. class B case but is not linear and accordingly is suitable only asa carrier amplifier. For three branches, the present invention affordsan efficiency of the order of 70%; for ten branches 80% as seen in Fig.11. A conventional class B amplifier has an efficiency of about 33% fora about a 50% eihciency for a class C carrier amplifier, plate-modulatedby a class B signal ampl'fier. and about a 60% overall efliciency forthe modulated wave amplifier exemplified by U. S. Patent No. 2,210,023of W. H. Doherty. However, the Doherty amplifier has the disadvantage ofa complicated load circuitwhich is operable only at a single carrierfrequency. The efliciency oi the three branch system illustrated in Fig.2 for the case of a signal modulated carrier wave is seen in Fig. 12 asof the order of 60%. By increasing the number of branches in thecircuit, and by careful choice of the ratio of maximum load ratings ofadjacent branches, the efliciency This compares with can be increasedmuch beyond that of such threebranch circuit. Fig. 12 shows acorresponding efllciency of about 78% therefor.

The impedance unit 21 indicated in Fig. 1, whereby the output circuitsof the several branch units arecombined to re-form the signal wave, maytake diflferent forms. The form illustrated in Fig. 2 comprises atransformer 30 which affords close coupling between the outputs of'therespective branch circuits. Such close coupling results in the amplifiedwave shapes A, B, C illustrated in Figs. 6, '7, 8 for corresponding wavesections A, B, C of Figs. 3, 4, 5. The matching of the power-amplifierunits to the load may be performed by an impedance-matching unit whichgives aclose coupling between each branch and the output load, andsubstantially zero coupling Fig. 14 shows between the respectivebranches. one embodiment for the latter type impedancecoupling unit.This unit comprises bridge or hybrid-coil circuits. Bridge unitcomprises four arms 56, 51, 58, 59 across which the output circuits ofbranches I and II are connected. The

second bridge unit 60 is connected across the terminal of one of thebridge arms 59. A diagonal pair of terminals of unit Wis coupled to theoutput circuit of branch III. Bridge 60 comprises arms 61, 62, 63, 64.The load 55 is shown coupled to bridge arm 54 through coupling coil 65.Load may, however, be connected directly in bridge 60, replacing arm 64thereof. Where more than three branches are used, additional bridgeunits are employed connected in a similar manner to that shown for threebranches in Fig. 14. Also, it only two branches are used in the system,a single impedance bridge unit such as 55 is sufllcient.

Figs. 15, 16 and 17 illustrate the resultant amplified wave forms A",B", C", corresponding to the respective divided signal wave forms A, B,C, produced at the signal-shaping units in a manner described inconnection with Figs. 3, 4 and 5. The wave shapes A, B, C' are of thesame wave form as the original signal-wave sections A, B, C. In thiscase the amplified waves comprise squaretop horizontal pulses, ratherthan the vertical narrow pulses produced by the transformer coupling,

as shown at A, B, C in Figs. 6, 7, and 8. The amplified wave forms A",B", C" are recombined at the load 65 to reconstruct the original waveforms as will now be evident to those skilled in the art. In practice,this alternative form of circuit for the output-impedance unit resultsin about the same efilciencies for sine-wave amplification as thevertical-division coupling system of Fig. 2 provided by transformer 30.For a sinemodulated carrier wave, the resultant efllciency is of theorder of 5 to 10% greater than that given by the system of Fig. 2.

Fig. 18 is a schematic diagram illustrating a number of generalapplications of the fundamental aspects of my present invention. Thesystem of Fig. 18 is applicable for signal-modullating a carrier wave.Separate sets of branch amplifiers are indicated for both the carrierwave and the signal wave. Referring particularly to Fig. 18, thecarrier-frequency source is indicated at 10, the output of which isconnected to the inputs, of a plurality of signal-shaping circuits-1|,12, 13. A three-branch amplification circuit is indicated, and thesignal shapers may correspond to those shown in Fig. 2, or modificationsthereof, as indicated hereinabove. The output of each of the signalshapers 1|, 1.2, 13 is connected to respective power amplifiers 14, 15,16. The amplifiers 14, 15, 15 correspond to those set forth inconnection with Fig. 2. The outputs of ampli flers H, 15, 16 are inturnconnected to impedance unit 11. Impedance unit 11 corresponds to unit 21in Fig. 1, 3D in Fig. 2, or may be an impedance circuit as shown in Fig,14. The output of impedance unit 11. is connected to the load generallyindicated at 18.

The signal wave which is to modulate the carrier wave is in turnamplified by a three-branch arrangement comprising signal shapers 8i,8!, 83 directly coupled to source 80. The signal shapers 8!, 82, 83 arein turn connected to the power amplifiers 84, 85, 86. The outputs of therespective amplifiers 84, 85, 86 are in turn connected in a suitablemodulating circuit connection to respec tive amplifier units 14, 15, 16.In accordance with the general arrangement provided by the system ofFig. 18, the carrier wave is split up into a plurality of signalsections by the signal shapers and correspondingly amplified at 14, 15,

' 16. The modulating signal 80 is also split up into a number ofsections in accordance with the principle of the invention, whichsections are amplifled at 84, 85, 86 and in turn used to modulate theamplified carrier sections at 14, 15, 16. The component modulatedsections are combined at impedance unit 11 to form the resultant desiredsignal-modulated carrier wave, which signal is connected to the loadcircuit 18.

If it is desired substantially to decrease the undesired harmonic signalcomponents, a, degenerative or negative feed-back circuit arrangement isprovided for the system as already mentioned.

Such negative feed-back circuit is indicated in the system of Fig. 18and comprises an isolation circuit unit 81 connected to a demodulator 88which in turn is coupled to the signalspurce 80 by an impedance-matchingunit 89. It is noted, how-.

ever, that many applications of the invention are commercially practicalwithout such negative feed-back arrangement.

The system generally indicated in Fig. 18 is in practice capable ofseveral different operative arrangements as follows. The arrangementwhich provides the greatest overall efficiency is that in which thecarrier and the modulating signal are split up into a number of branchesin accordance with the fundamental principles of the invention and asindicated in Fig. 18. and

wherein the amplifier branches of the signal 80 at 84, 85, 8B arearranged to plate-modulate a corresponding amplified carrier-branchsignal at 14, 15, 1B. The resultant wave shape at outputs 11, 18 willnot be the conventional signalmodulated wave shape, but will have thesame power and frequency content which, when demodulated by any type ofdetector, will provide the identical signal output as the conventionaltype. Experimentally, I found that the efficiency of such asignal-modulated system is of the order of 70% using a 3 db. ratio ofsignal division between the respective branches. It is to be understoodthat the number of amplifier branches for either the carrier or themodulator signal is not necessarily three, but may be more or less inaccordance with the desired design characteristics. Also, the signaldivision need not be 3 db. as set forth generally hereinabove inconnection with Fig. 2. Also, it is not requisite that the carrieramplifier contain signal shapers. Instead class C amplifiers operatingin parallel may be used, which amplifiers are in turn modulated by theinvention-type signal-divided amplifiers. In this latter case I havefound that the power-conversion efficiency is of the order of 60 in oneembodiment.

Furthermore, another arrangement is effected by branch amplifyingsignal-shaped sections of a constant-voltage carrier wave, butconventionally amplifying the modulating signal by a number of parallelamplifiers. The output of the signal amplifiers is then connected to thecarrierbranch amplifiers for the modulation. It is to be understood thatit is not necessary that a like number of amplifiers be used for carrierand signal source amplification. In Fig. 18 each branch of the signalamplifier is shown as modulating a single branch of the carrieramplifier. However, this is merely schematic and not requisite. Severalbranches of the signal amplifier may be combined by an output circuitand used to modulate a single branch of the carrier amplifier. Also, onebranch of the signal amplifier may modulate several branches of thecarrier amplifier. The only requirement is that the maximum amplitude ofeach signal pulse must be great enough to give the required depth ofmodulation, ordinarily to the associated carrier branch or branches.

Other modifications for operating the system of Fig. 18 will be apparentto those skilled in the art. For example, instead of employingplate-modulations between the signal-amplified branches and the carrierbranches, so-called lowlevel modulation may be used. Thus, each carrierbranch is modulated by a signal branch at either the control-grid,suppressor-grid or screengrid electrodes of the respective power tubes.A further equivalent of this arrangement is to similarly modulate 0rplate-modulate the signalshaper or drive stages preceding thepower-amplification stages. results. with the low-level the platemodulation. a

An extension of the principle of my present invention will now bedescribed in connection with Figs. 19 to 22. This is a method ofamplifying a signal-modulated wave which, when used with the circuit ofFig. 2,results in an output having a considerably reduced D. C. powerrequirement as compared with conventional types of modulation systems.The modulated wave signal is schematically indicated by curve a, Fig.19. Signal a is rectified and filtered to produce a directcurrent pulsehaving a wave shape corresponding to the envelope of curve a. Fig. 20shows the rectified pulse at b. The uni-directional current pulse,corresponding to curve b, is connected in series with the original wave,corresponding to curve a, resulting in wave c shown in Fig. 21. Thesignal c contains all the components of the original wave, but isuni-directional in nature, and is not an alternating current. Itsamplitude is zero when no modulation is present. The currentcorresponding to wave 0 is then applied to the system of Fig. 2 at theposition corresponding to signal source 20. Plate current proportionalto curve 0 will flow upon application of signal 0 through the system.The output at 3i will appear as indicated by curve d, Fig. 22. It is tobe note that the horizontal scale of Fig. 22 is expanded for clarity.Similar operation is efiected when signal ais a modulating signal, andthe system used as a signal modulator of a carrier wave.

In the system just described,.there is no output energy from thesignal-modulated carrier amplifier until the signal is applied. Asindicated by Fig, 22, the carrier is completely modulated for all'levelsof the signal wave. Thus it is seen that the carrier level isautomatically adjusted A somewhat lower 'eflleien'cyf modulationas'against} .tion in the carrier level.

so that its peak amplitude is always just equal to the peak amplitude ofthe signal wave. The advantages of such an amplification method for thesystem of Fig. 2 are more appafent at modulations below 100%. However, alarge overall reductionin input power is obtained by a reducknown thatsuch reduction is by a factor lying between 0.80 and 0.88. When thisfactor is combined with the considerable improvement and efficiency at100% modulation that the present invention makes possible, it is foundthat the power input for a variable signal modulated carrier wave with aspecified peak value is less by a factor of about tol over aconventional system. Such resultant advantages will accrue where thecarrier-wave modulation is by telephonic currents, and possibly also bytelegraphic pulses of audio-frequency energy.

The design technique of rectifier and filter circuits which provide auni-directional bias at all times equal to the peak value of the inputsignal wave is a well known and highly developed art. No design oroperating difliculties of unknown character are involved. Circuits ofthis type have been in wide use infilm-recording systems for many years,and the novelty of the present invention involves their application to asignal-modulated carrier amplifier of high ef- Y ficiency, in such wayas to render the input power substantially proportional to the amplitudeof the output wave. For a description of such variable-bias circuits infilm recording, reference is made to the Journal of the Society ofMotion Picture Engineers of February 1942, pages 125-147.

Variations of the principles of the present invention and theirapplication in practice will become evident to those skilled in the art.The

significant advantages of the linear amplification performed by thepresent invention reside in the substantial increase in power-conversionefficiency of direct-current input to alternatingcurrent output. Suchefficiency improvement in practical engineering design is much greaterthan the actual percentage difference might appear to convey. Themaximum power handling capacity of a given vacuum tube ordinarily islimited by maximum permissible values of average and peak platecurrents, peak plate voltage and average plate-power dissipation. On thefactor of plate-power dissipation alone, if its efiiciency is increasedfrom 50% to 75% by the invention system, the power handling capacity ofa vacuum tube is doubled. Also, since in each of the branches of themultiple-path amplifier arrangement, the average plate current is muchcloser to the peak plate current than in a conventional amplifier, andthe peak plate voltages are the same, it has been found possible toobtain in apractical design embodying the disclosed principles aboutdouble the power output from a given tube complement as compared to aconventional system. This is of great advantage in high-power systemsfrom the standpoint of cost. In low-power mobile radio trans- For steadyspeech it is,

mitting equipment, it provides a marked improvement in space andfilament-power requirements.

The principles of the invention are particularly indicated foramplifierswith power outputs of about 100 watts and upwards. The applications ofthe invention principles are numerous, and are flexible as to frequencyranges. function, or power rating. The extra cost of sockets, smallcomponents and wiring for applications of my invention to systems ofover 100 watts is negligible, being not more than 1 to 5% of the total.Also, although the filamentpower requirements may be 25% greater due tothe larger number of tubes, the fact that the filament power is obtainedat high efliciency and low cost makes this difference unimportant. Whatis significant is that for a given output requirement, the inventionpermits an amplifier of about one-third, and a transmitter of aboutone-half, of the conventional cubic space and weight requirements. Thisis mainly due to the reduction in heat dissipation referred to. Also,

- this same factor will result in an improvement in tube life of theorder of 2:1. These factors do not include the large improvementsfurther made possible by the arrangements disclosed in Figs. 19 to 22.For radio transmitter power out- ;put above 1000 watts, the use of themultiplebranch carrier amplifier modulated by a multiple-branch signalamplifier as disclosed in connection with Fig. 18 is preferred. Fortransmitter power ratings between 100 and 1000 watts, a single-branchcarrier amplifier modulated by a multiple-branch signal amplifier or astraight modulated-wave multiple-branch linear amplifier is preferred.

The significant practical improvements made possible by the presentinvention may be further emphasized by a consideration f the following.

It is well known that the efficiency of a conventional class B amplifieris proportional to the signal amplitude so that its efilciency variesbetween 0 and for speech amplification. The aver-. age value of thespeech wave is, in standard engineering practice, considered as not morethan 25% of the peak value. Thus, the efliciency of such amplifier whenaveraged over a period of speech modulation is less than 0.25 70% orabout With an amplifier for this purpose designed in accordance with thepresent invention, overall efilciencies of '77 72% and 60% result forcorresponding peak sine-wave inputs of 0 50% and 25% relative signalvalue. Thus, an average speech efficiency of about 60% is realized ascompared to the 1'7 for the conventional arrangement. The sameconsideration for a signal-modulated carrier wave results in a 32%efilciency by the conventional class B amplifier, and 72% by theinvention multiple-branch amplifier.

An important commercial application of the invention amplifier is itsuse as a linear amplifier in the output stage of a radio-transmitter inwhich the carrier 'wave is suppressed and only the'side bandstransmittted. Considering a carrier wave of 100 watts, the total poweroutput of the transmitter when completely modulated by a signal wave iswatts, including carrier wave and side bands. For the varying degree ofmodulation that occurs during the telephonic transmission, the outputpower of a conventional transmitter varies between 100 and 150 watts,and if lowlevel modulation is used. then the varying modulated wave isamplified by a linear output stage. If, however, the carrier issuppressed by a bridge circuit or by a balanced modulator, then theoutput power from the transmitter varies between 0 and 50 watts. Thisrepresents a much wider amplitude range than the previous 100 to 150watts and therefore is a case in'which the improvement of themultiple-branch amplifier over the conventional single-branch amplifieris very large.

The following is a tabulation comparing the relative efliciencies ofseveral'conventional output pliflers having signal output capacities inthe 1 ratio of about three decibels between adjacent systems withcorresponding ones employing the principles of the present invention:

- Efilciency Output power D-C input (watts) gi gggg relative tol(wietts) for mldtllfor modulation g casgullAtiior at on percen per can0 mo 8 on per cent of per cent oflA. Conventional plate-modulated system100 102 150 150 172 262 67 57.5 1.0 1.00 18. 3-branch plate-modulatedsystem 100 102 150 140 145 230 70 65.5 1.19 1.14 2A. Conventionalgrid-modulated system 100 102 150 300 300 300 33 50 0.50 0.87 213.3-branch grid-modulated system 100 102 150 167 215 264 47 57 0.80 0.993A. Conventional plate-modulated system with carrier control 0 6 150 058 262 12 57 3.24 1.00 38. 3-branch plate-modulated system with carriercontrol 0 6 150 0 18 230 33 65 9.55 1.14 4A. Conventional grid-modulatedsystem with carrier suppression 0 2 60 0 30 150 6 33 5. 72 l. 75 4B.3-brench grid-modulated system with carrier suppression 0 2 50 0 6 70 3370 28. 7 3; 74

The present invention may also be used to materially improve theefiiciency in electronically generating a radio wave by the oscillatorprinciple. An oscillator is essentially an amplifier which obtains itsinput wave as a portion of the output power. Thus, the circuit of Fig. 2can be made to oscillate by feeding part of the output energy fromsecondary winding 54 back into the input circuit of the amplifier atsignal source 20, in the correct phase. An efiiciency of the order of65% is realized in a conventional class C oscillator. Using similartubes for a three-branch circuit with 3 db. separation between adjacentbranches, an oscillator efiiciency of more than 75% is obtainable.

Fig. 23 diagrammatically indicates such an oscillator. The frequencysource for the oscillator is indicated at 90, and may be any suitabletype of signal generator such as quartz crystal, vacuum tube, or thelike. The output of signal source 90 is impressed upon the input of amultiple-branch amplifier 9| constructed in accordance with theinvention. The three-branch amplifier indicated at 9| may 'wellcorrespond to the amplifier of Fig. 2. The output of amplifier 91 isimpressed upon output circuit 92. A portion of the output energy at 92is fed back to the input of amplifier 9 I, as schematically indicated bylead 93. Such feed-back is in proper phase so as to combine with thesignal from source 90, and results in the high efiiciency oscillatoroutput at terminals 94.

Although the invention has been described in connection with variousapplications, modifications and exemplifications, it is to be understoodthat further embodiments and variations thereof, falling within thebroader spirit and scope of the invention, may be practiced by thoseskilled in the art, and accordingly it is not intended to be limitedexcept as set forth in the following claims.

What is claimed is:

1. An amplifying system comprising a plurality of amplifiers, meansconnecting the input of each of said amplifiers to a signal source, saidmeans having elements for predetermining the mag- "nitude levels of thesignal impressed upon each amplifier, a translating unit connecting theoutput of each amplifier to a load circut for combining the amplifiedsignal portions into the original signal wave form, said unit comprisingbridge circuits arranged to effect substantially zero coupling betweenthe outputs of the respective branches while effecting substantialcoupling between each amplifier and the load circuit.

2. An amplifying system comprising an alternating signal source, aplurality of class B amwhile effecting substantial coupling between eachamplifier and the load circuit.

3. An amplifying system comprising a plurality of amplifiers, each withsubstantially the same phase shift, means connecting the input of eachof said amplifiers to a signal source, said means each withsubstantially the same phase shift and having elements forpredetermining the magnitude levels of the signal impressed upon eachamplifier, and a translating unit connected to the output of eachamplifier for combining the amplified signal portions into the originalsignal wave form.

4. An amplifying system comprising a signal source, a plurality ofamplifiers, each with substantially the same phase shift, signal shapingmeans individually connecting the input of each of said amplifiers tosaid signal source, said means each with substantially the same phaseshift and having elements for predetermining the magnitude levels of thesignal impressed upon each amplifier, and an impedance unit connected tothe output of each amplifier for combining the amplified signal portionsinto the original signal wave form.

5. An amplifying system comprising a signal source, a plurality ofamplifiers, each with substantially the same phase shift, signal shapersincluding circuit connections between the input of each of saidamplifiers and said signal source, each of said signal shapers withsubstantially the same phase shift and including an electronic path withbiasing elements for predetermining the magnitude levels of the signalimpressed upon each amplifier, and a translating unit connected to theoutput of each amplifier for combining the amplified signal portionsinto the original signal wave form.

6. An amplifying system comprising a. plurality of linear amplifiers ofdifferent signal output capaci ies, each with substantially the samephase shift, means including circuit connections individually connectingthe input of each of said amplifiers to a signal source, said means eachwith substantially the same phase shift and having elements forpredetermining the magnitude levels of the signal impressed upon eachamplifier in correspondence with its output capacity, and animpedance-matching unit connected to the output of each amplifier forcombiningthe amplified signal portions into the original signal waveform.

7. An amplifying system comprising an alternating signal source, aplurality of class-B amplifiers having signal output capacities in theratio of about three decibels between adjacent amplifiers, each withsubstantially the same phase shift, signal shaping means individuallyconnecting the input of each of said amplifiers to said signal source,said means each with substantially the same phase shift andpredetermining the magnitude levels of the signal'impressed upon eachamplifier in correspondence with its output capacity, and an impedanceunit connected to the output of each amplifier for combining theamplified signal portions into the-original signal wave form. a

8. An amplifying system comprising an alternating signal source, aplurality of class B am plifiers having signal output capacities in theratio of about three decibels between adjacent amplifiers, each withsubstantially the same phase shift, signal shaping means individuallyconnecting the input of each of said amplifiers to said signal source,said means each with substantially the same phase shift and includingelectronic paths for predetermining the magnitude of a signal portion tobe impressed upon each amplifier in accordance with its output capacity,and a transformer connected to the output of each amplifier forcombining the amplified signal portions intothe original signal waveform.

9. An amplifying system comprising a plurality of'amplifiers, each withsubstantially the same phase shift, means connecting the input of eachof said amplifiers to a signal source, said means each withsubstantially the same phase shift and having elements forpredetermining the magnitude levels of the signal impressed upon eachamplifier, a translating unit connected to the output of each amplifierfor combining the amplified signal portions into the original signalwave form, and a negative feedback circuit responsive to the outputs ofsaid amplifiers and in circuit connection to the inputs thereof forsubstantially suppressing harmonic distortion in the combined signalr10. An amplifying system comprising an alternating signal source, aplurality of class B aniplifiers having a signal output capacities inthe ratio of about three decibels between adjacent amplifiers, each withsubstantially the same phase shift, signal shaping means individuallyconnecting the input of each of said amplifiers to said signal source,said means each with substantially the same phase shift and includingelectronic paths for predetermining the magnitude of a signal portion tobe impressed upon each ampiifier in correspondence with its outputcapacity, a translating unit connected with the output of each amplifierfor combining the amplified signal portions substantially into theoriginal signal wave form, and a negative feedback circuit responsive tothe output of said amplifiers and in circuit connection to the inputsthereof for substantially suppressing harmonic distortion in thecombined signal.

7 CHARLES B. FISHER.

